Various types of office machines, particularly typewriters having electrically controlled paper insertion mechanisms, output printers of word processors, computers and the like, are known in which paper is fed to a platen either from a roll or from a zig-zag folded supply. These papers frequently have perforated edges for engagement with a suitable sprocket. Such edge perforations are undesirable in many applications, and apparatus has been developed to cut off the perforated edges. Such cutting operation is comparatively complex, requires additional machinery, and eventual handling for disposal of the cut-off strips.
The increasing use of automatic typewriters, word processors, and the like, makes it desirable to permit output printing to be carried out directly on single sheets. Such single sheets may have letterheads printed thereon, format information required for billing, and the like. Supply of single sheets to typewriter apparatus, which can operate automatically, is difficult since, if single sheets are fed to the typewriter sequentially, inaccuracies with respect to feed of any one sheet become additive. The sheets, although theoretically all of the same size, are subject to tolerances. Slip in the feed, likewise, may become additive with respect to tolerance inaccuracies, so that additive errors of individual line alignment may become troublesome.
It is known and has been previously proposed to feed sheets individually and sense the supply position of the leading edge of the sheet before feeding the sheet. Such an arrangement, while suitable and well known with large printing machines, is difficult to combine with typewriter or typewriter-type printing apparatus, since automatic typewriters and the like have insufficient room to locate the required stop apparatus and the sensors therefor. The space to located photo cells and the like is limited, and the reciprocating movement of a printing element, such as a "daisy-wheel", a jet printer or the like, additionally interferes with use of photo electric apparatus to check proper alignment of sheets which have been supplied or are to be supplied to the printing apparatus. Photo-sensitive units, in which the edge of the sheet interrupts a light beam, thus are practically not applicable due to space limitations. Using the foreward edge of a sheet to sense its position by reflection of a light beam has the disadvantage that the reflection of sheets which may be colored or carry pre-printed subject matter at the upper edge is insufficient to permit response of commercial sensing apparatus. The difference in brightness of reflected light is frequently insufficient in order to permit accurate sensing of the front edge of a sheet.
It has previously been proposed to provide a sheet feed apparatus in which a separate drive motor supplies sheets to the platen of a typewriter--see, for example, the referenced British Pat. No. 1,569,370, by the inventor hereof. Use of a separate drive motor, while suitable to supply sheets, is costly, and synchronization of the drive motor with the rotation of a sheet-carrying and sheet supply platen causes difficulty.